The present invention relates to novel liquid compositions for providing silica coating films on various substrates, methods for the preparation thereof and processes for forming silica coating films on a substrate with the same.
Formation of silica coating films on various substrates is usually performed by chemical vapor-phase deposition or by liquid coating. The former method has obtained wider prevalence but this method is defective in several aspects including, for example, the necessity of special apparatus, limitations in the dimensions of the substrate articles, difficulty in mass production, minimum applicability to the formation of mixed coating layers of an organic substance and an inorganic substance and the like. Therefore it is a recent trend that the method of liquid coating is replacing the vapor-phase deposition method more and more owing to the simplicity of the process.
The liquid coating method however, is not free from several problems such as the tendency of precipitation of the particulate ingredients in the coating liquid prepared by dispersing powders of silica or a glass in a solvent with a soluble polymeric substance as the suspending agent or a binder, unevenness in the thickness of the coating films and occurrence of pinholes in the coating films, especially, when the coating film is thin. As a result, the method is not always satisfactory as an industrially practicable procedure. In order to overcome these problems, a method has been proposed in which the powdery silica is replaced with the reaction product of a carboxylic acid, a halogenated silane and an alcohol in the formulation of the coating liquid (see, for example, Japanese Patent Publication Nos. SHO 52-16488 and SHO 52-20825). Although this method presents improvements in the precipitation of the particulate ingredients in the coating liquid and unevenness in the thickness of the coating films, it is subject to another problem that the coating liquid necessarily contains undesirable halogenated compounds as by-products such as hydrogen halides, e.g. hydrogen chloride, and carboxylic acid halides, e.g. acetyl chloride. These halogenated compounds cannot be removed completely because presence of the halogenated compound is indispensable for the desired reaction to take place whereas an excessive amount of the halogenated compound affects the stability of the coating liquid adversely leading to undesirable gelation of the coating liquid during storage so that the concentration of the halogenated compounds in the coating liquid must be controlled within a narrow range during the reaction. In addition to the above inconvenience, the presence of the halogenated compounds in the coating liquid after completion of the reaction causes corrosion of the substrate surfaces during coating or subsequent heating steps.
There has been a great demand, therefore, for a coating liquid containing no halogenated compounds which still permit the necessary reaction to proceed smoothly in the coating liquid.